After testing two fins from the same mold with different fillets i'm convinced the fillet will do 40nts , the fins were 55deg deltas, the bigget difference i found was the ability to point, i could do a good run off the wind and hit some decent chop and sail back to the launch in one tack, the second was the ability of the fin to recover,usually when a delta lets go it's gone.
The wind was 20 to 30nts using a naish sp80 and 5/8 ev05 ,and a 17cm thicker fillet & couldn't crack 36 changed to the same fin with thinner fillet and 36.4 , my guess is in the rite conditions with a lucky gust a fillet will do 40nts ??

From what I could find in the research, the radius of the fillet should be 6 - 10 % of the chord. There is a small advantage in making the fillet a more complex compound curve in the leading edge area, but the researchers concluded the effort vs benefit was not worthwhile. Most of the benefit occurs in the first third. A fine fillet edge is important, hence the efforts to reduce the gasket width, and finishing the edge to as fine as the material and longevity will allow.

These measures are to increase efficiency and reduce drag by taming pertubed flow patterns in order to make a small surface area fin with improved lifting behaviour, not so much a speed fin. I don't know, but I suspect that at very high speed, the interference pressure waves between fin and board will overwhelm the fillet effect. I am finding it hard to find research on the hydrodynamics of turbine end plates at higher velocities - after all, the turbine engineers are after maximum lifting (power generation) efficiency, not how fast they can get the turbine to spin. I think I will need to follow Yoyo's lead and look at high speed propellor design.

The flip side: the small surface area, the friendly and confidence inspiring handling characteristics of filleted fins could be enough to see them cross the 40 knot mark. I certainly have sailed faster downhill over chop with a filleted fin than with a Delta, simply because I am confident its not going to let go. As Decrepit says, time for some more experiments!

fangman said..

The flip side: the small surface area, the friendly and confidence inspiring handling characteristics of filleted fins could be enough to see them cross the 40 knot mark. I certainly have sailed faster downhill over chop with a filleted fin than with a Delta, simply because I am confident its not going to let go. As Decrepit says, time for some more experiments!

I think the reason you can sail further off the wind is because the angle of the fillet directs the water flow to the angle of attack , where as the standard delta's leading edge is thin opposed to the chord and easy for the water flow to be directed to the opposite side of the fin , maybe to be honest I don't know

Peter 30 Grit! I didnt know that existed - is a flap wheel? Do you reckon its that faster than a 7 inch grinding wheel?
Today's product, Bling finish for Pepe.

decrepit said..
So Keef, you've hit on an important point here, the ideal size of fillet, too small may not work as well, too big probably slower.
Is it a fixed size for all fins, or is it a percentage of chord or depth????????

The 17cm with the larger fillet was getting lifted in the middle of the speed run, the smaller fillet settled the board down and faster My guess is the smaller the fin the bigger the fillet ,i couldn't image a 28cm fin with the same fillet proportion as a 17cm with and over sized fillet as the fin would be huge
I would say there would be a cut off size (maybe 24), a few years back when i first had a look at something like a fillet the initial idea was to fill the gap of the fin and reduce the 90deg angle of the base,so with the larger fins thats maybe all you would need

waricle said..
My bad, is wasn't 30 grit it was 24

Thanks Peter, I will keep an eye for those and try them out if it is quicker than grinding.

waricle said..
Does this Fin make me look fat?

Defo your best angle Nice job!

keef said..
The 17cm with the larger fillet was getting lifted in the middle of the speed run, the smaller fillet settled the board down and faster My guess is the smaller the fin the bigger the fillet ,i couldn't image a 28cm fin with the same fillet proportion as a 17cm with and over sized fillet as the fin would be huge
I would say there would be a cut off size (maybe 24), a few years back when i first had a look at something like a fillet the initial idea was to fill the gap of the fin and reduce the 90deg angle of the base,so with the larger fins thats maybe all you would need

sounds like the 17cm thin fillet is ready to get the team rider out testing it Keef

fangman said..
>>>
My current plan insert the delrin rod and then drill and tap both the aluminium and delrin at the same time. That way the aluminium can do the bulk of the work and the delrin is only there as a backup should there be a corrosion issue in future.

Good thinking 99, I remember thinking the same thing for one of my fins, but then discovered that I'd drilled the holes too big to thread. Fiberglass doesn't hold a thread as well as alli anyway, so I didn't stress over it.

Blimey Swindy you have more fins than I do! That looks great. I will be really interested in your experience on your '22'. If we need to trim some off the trailing edge, I can easily make you some scaled down templates of the foil for you to use and shape to.

As there is very little difference in surface area atm I probably will do further mods so templates will be great, thanks Ross. I will give it a couple of goes first though.

Swindy said..
Used the new 22 briefly yesterday and as I suspected still felt a bit big so I attacked it with the grinder again. My plan was to remove
The only reason I am doing this is because for my purposes the gap between the 20 and 24 is too substantial and the fangman is unwilling to risk certain death by investing in a new print and mold for a real 22. All 3 available sizes are fantastic and the gap between the 24 and 28 isn't an issue.
if enough interest is expressed in a real 22 we may be able to persuade him.

....Or, you can just send them to Swindy and he will happily cut back a 24 into a super nice looking 22. Simples.

Looks really nice mate, I am very keen to see it in the flesh, because you are dead right, I really need to do a 22. I noticed it yesterday, my 24 was too big and the 20 just a tad too under-done for uphill work on the 6.6.

decrepit said..
And then somebody will want an 18.
I was on a 16.5 yesterday, a tad small for my 5.4 uphill but nice downwind. I should have been on my 18 to be comfortable in both directions.

Now you come to mention it Mike, for them realy big days. That can go on the wish list for now though. The 22 will be my go to fin on the 100L and occasionally on the 90L.

Swindy said..
....as the managing director is too busy smashing personal bests and ripping up weed with his product.

...as well as feeding the Minister For Excessive Windsurfing Expenditure massively over inflated sales figures in order to try and get the FF22 design process off the ground.
Good work on that FF22 Swindy, as soon as I get a chance I will copy it

I think it's much truer for a fangy than a delta. But we're not comparing apples with apples here. The pointer is going to win in deep water chop, no doubt about it. But the fangy fin will win hands down in thick surface weed. How the fangy compares to a good conventional weedy is more problematic. However hardie used a pepe weed 48deg and a fangy on the same day and found them very similar, but I've no idea about their area or sizes.
There's also the issue of leverage, a long narrow fin will have more leverage than a short wide one, even if they both produce the same lift. So there's going to be a difference in board trim.